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Hong Kong has a higher level of background radiation than Tokyo, says a group of citizen scientists who hope to allay fears that travellers to Japan are at greater risk of exposure.

Non-profit group Safecast has been gathering data on radiation levels in Japan since the 2011 earthquake and Fukushima nuclear disaster.

They have been to Hong Kong several times, driving around in cars with Geiger counters to measure the radiation levels in some of the city's most heavily populated areas.

And they found that Hong Kong's level of background radiation is around 100 counts per minute, about three to four times higher than Tokyo's, says Pieter Franken, a computer scientist and volunteer radiation monitor based in Tokyo.

"It's likely the granite used in construction," said Franken, who set up the monitoring group with others as they felt radiation data being provided by the Japanese government was inadequate.

Even in a hotel room on the 15th floor, the radiation levels were still higher than in most Asian cities, he said.

The Hong Kong Observatory said that based on its own measurements, the level of background radiation in the city did not pose a risk to human health.

Research by the Observatory and Polytechnic University in the late 1990s found concrete used in the city contains radioactive elements - probably from the granite used as aggregate.

Granite contains minerals with traces of uranium, thorium and radium, which give off gamma radiation.

There are three types of radiation - alpha, beta and gamma. Alpha rays cause damage only when particles are inhaled or swallowed, as human skin can block the radiation. But beta rays can enter the skin, and gamma rays can pass through the body.

Some radiation damage can be repaired by the body, but receiving 1,000 millisieverts - the measurement used per "dose" - in an hour would cause radiation sickness. Receiving 10,000mSv would kill in a matter of weeks.

The worldwide average background radiation dose for a year is 2.4mSv, and it ranges from 1mSv to 10mSv in the countries tested by the United Nations Scientific Committee on the Effects of Atomic Radiation.

The good news is that Hong Kong's annual background dose, as measured by Safecast, is about 2.4mSv - the same as the global average.

The Observatory monitors the Daya Bay nuclear power plant to make sure it is not causing radiation levels to rise in the city. But the buildings, development and health bureaus all say checking building radiation levels is not within their remit.